Magnetic tunnel junction (MTJ) elements serving as magnetoresistive elements have a multilayer structure including a storage layer, in which the magnetization direction is changeable, a reference layer, in which the magnetization direction is fixed, and an insulating layer disposed between the storage layer and the reference layer. The MTJ elements are known to have a tunneling magnetoresistive (TMR) effect, and used as storage elements of memory cells in magnetic random access memories (MRAMs).
MRAMs store data (“1”, “0”) based on changes in relative angle between magnetization directions of magnetic layers included in each MTJ element, and are nonvolatile memories. Since the magnetization may be switched in several nanoseconds, data may be written and read at a high speed. Therefore, the MRAMs are highly expected as next-generation high-speed nonvolatile memories. The cell size of the MRAMs may be reduced by employing spin transfer torque magnetization switching, in which the magnetizations are controlled by means of spin polarized currents. The reduction in cell size may lead to an increase the current density. The increased current density may allow magnetization switching in storage layers to be performed more easily. Therefore, MRAMs with high density and low power consumption may be obtained.
In order to improve the density of nonvolatile memories, the magnetoresistive elements should be integrated more densely. However, thermal stability of ferromagnetic materials, which form magnetoresistive elements, may be degraded if the entire device size is reduced. Therefore, improvement in the magnetic anisotropy and the thermal stability of the ferromagnetic materials is a problem.
In order to solve this problem, attempts have recently been made to produce MRAMs including perpendicular magnetization MTJ elements, in which the magnetizations of the ferromagnetic materials are perpendicular to the film plane. The magnetic materials to form perpendicular magnetization MTJ elements need to have perpendicular magnetic anisotropy. In order to achieve the perpendicular magnetic anisotropy, materials having crystalline magnetic anisotropy or interface magnetic anisotropy are selected. For example, FePt, CoPt, and FePd have strong crystalline magnetic anisotropy. A number of MgO-based MTJ elements including MgO as a tunnel barrier, and ferromagnetic layers disposed with the MgO sandwiched therebetween and having interface perpendicular magnetic anisotropy, such as layers of CoFeB, are reported.